The present invention relates to superalloys having increased strength and thermal stability at room and elevated temperatures. More particularly, the present invention relates to a thermomechanical process involving rotoforging for producing superalloys with superior mechanical and thermal properties.
Superalloys such as nickel-, iron-nickel- and cobalt-based alloys have long been known and used in high temperature applications (at temperatures generally above 540xc2x0 C. (1000xc2x0 F.)). Such alloys have been particularly useful in the construction of aircraft engine components because of the operating requirements for strength and the ability to resist loads for long periods of time at elevated temperatures. These alloys are also used in electron beam generating devices, such as x-ray tubes, which also operate in high temperature and high mechanical stress environments.
X-ray tubes are typically comprised of opposed electrodes that are enclosed within a cylindrical vacuum vessel. The electrodes, in turn, comprise a cathode assembly, which emits electrons and is positioned at some distance from the target track of a rotating, disc-shaped anode assembly. The target track or impact zone of the anode is typically constructed from a refractory metal with a high atomic number and melting point, such as tungsten or tungsten alloy. The cathode has a filament which emits thermal electrons. The electrons are then accelerated across the potential voltage difference between the cathode and anode assemblies, impacting the target track of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation or x-rays, while the balance is converted to thermal energy or is contained in back scattered electrons. The thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is ultimately removed from the vessel by a circulating cooling fluid. The back scattered electrons further impact on other components within the vacuum vessel, resulting in additional heating of the x-ray tube. The resulting elevated temperatures generated by the thermal energy subject the x-ray tube components to high thermal stresses which are problematic in the operation of the x-ray tube.
Additionally, because of the very high temperatures at the target plane of the anode, it is important that the alloys located in close proximity to the target plane be fabricated in such a manner to withstand the elevated temperatures and thermal stresses. Alloy that is typically used in x-ray tube components is designated as Alloy 909 and known by trade names Incoloy(copyright) 909 (manufactured by Inco International, Huntington, W.Va). and CTX-909 (manufactured by Carpenter Alloys, Reading, Pa). Although their compositions are substantially the same, Incoloy(copyright) 909 and CTX-909 exhibit different microstructural characteristics which will be discussed in greater detail below.
Alloy 909 is a controlled, low thermal expansion alloy that is typically used at temperatures not higher than 700xc2x0 C. (1292xc2x0 F.). Alloy 909 is manufactured in the form of an ingot using vacuum induction melting (VIM) and vacuum arc remelting (VAR) process. A wrought bar is then made from the ingot by a hot rolling process. Small diameter alloy bars and rods that are used for fastener applications are usually made from a cold drawn wire.
According to Aerospace Material Specification (AMS) Guidelines 5884, the material properties of Incoloy(copyright) 909 are quite sensitive to the thermomechanical treatment received during processing of the alloy. AMS 5884 specifies grain size requirements for alloys such as Incoloy(copyright) 909 in industrial uses, and non-conformance with these requirements results in rejection of the alloy. Any cold work that is performed on Incoloy(copyright) 909, for example, cold drawing of the wire, requires a re-solution and precipitation heat treatment. Re-solution annealing is one of the critical steps in controlling the grain size, and subsequent material properties of the alloy. It is recommended that the re-solution annealing be performed at about 982xc2x0 C.xc2x114xc2x0 C. to avoid excessive grain growth. If this temperature exceeds the recommended limits, rapid grain growth occurs, resulting in a reduction in the strength of the alloy.
Rejection of alloys due to non-conformance of the grain size is unfortunately quite common. Re-working of the alloy is usually to be avoided, since an additional cold drawing step performed above a critical deformation level, often changes the final dimensions of the alloy bar. Additionally, alloys such as Incoloy(copyright) 909 and CTX-909 are custom fabricated by their individual manufacturers. The conventional process is lengthy, with a typical delivery cycle of bet ween six months and one year. Further, the end user must typically order a whole mill run, even when only a small quantity is desired. The lengthy manufacturing time and limited availability of the alloys create serious problems for the end users for several reasons. First, the user must anticipate his/her needs well in advance, yet may still fall short of the needed quantity of the alloy. Second, current processes do not allow the end user to rework a larger size alloy bar stock into a smaller size. Modification is generally performed by the alloy manufacturer. There, thus, remains a need to provide a more efficient process for producing high strength and thermally stable alloys of a desired size for use in high temperature applications.
The present invention is directed to a thermomechanical method for producing alloys with increased tensile strength and thermal stability. The method of the present invention further provides a means of fabricating smaller size alloy bars and rods with greater flexibility than those produced by conventional methods. The method involves heat treating and then rotoforging the alloy material at a sufficient deformation level and temperature to fragment the grain boundary phases of the alloy. Subsequent precipitation age-hardening results in an alloy having increased tensile strength at room and elevated temperatures (xcx9c649xc2x0 C.), good ductility, and excellent stress-rupture characteristics. The thermomechanically treated alloy is characterized by a micro structure exhibiting an ultra-small grain size of about 7 microns or less in diameter, fragmentation of the grain boundary phases, and dispersed carbides inside the grains.
Rotoforging has not heretofore been applied or considered in the fabrication of small diameter alloy bars and rods, and provides a means of producing smaller size alloy materials from larger sized alloy material. This feature is particularly beneficial in overcoming the production problems that consumers typically face with existing manufacturing processes. With only two producers of Alloy 909, the consumer must typically order a whole mill run, even when the quantity desired is small. Further, the delivery cycle is quite lengthy (typically 6-12 months) and, as a result, the availability of the Alloy 909 is frequently limited. The thermomechanical method of the present invention overcomes these problems by providing a means for the consumer to forge alloy materials to a desired size and quantity. The present method can be used to produce new and improved alloys having comparable superior mechanical and thermal properties for use in high temperature applications including, but not limited to, jet engines, x-ray generating devices, gas turbine components such as combustion blades and vanes, etc.